1. Technical Field
The present invention relates to a magnetic recording medium such as magnetic tape.
2. Background Technique
A problem with a coated type of magnetic recording medium including a magnetic layer having ferromagnetic powders dispersed in a binder is that the constituents forming the magnetic layer such as ferromagnetic powders come off during repeated running.
As recording density increases, it is required to increase recording frequency and, hence, to place the gap between the magnetic layer and a magnetic head under precise control. So far, attention has been paid only to detachment of powders from the overall surface of the magnetic layer in sliding contact with a magnetic head. However, it has now been found that even a slight amount of powders coming off the edges of tape has an adverse influence on the gap between the magnetic recording tape and the associated magnetic head. For instance, this makes an error rate likely to increase in recording digital signals on digital compact cassette (DCC) or other tape, and so offers a problem in view of practical use. There is thus a strong demand for the development of a magnetic recording medium which makes powders more unlikely to come off than ever before.
Various proposals have so far been made to prevent powders from coming off. For instance, JP-A 1-118213 proposes to incorporate in a binder used for a magnetic layer polyurethane resin having a glass transition temperature Tg of up to 0.degree. C. in combination with polyurethane resin having Tg of 50.degree. C. or higher and containing a polar group in its molecule. By the combined use of the soft polyurethane resin having Tg of up to 0.degree. C. and the resin having Tg of 50.degree. C. or higher and containing a polar group it is possible not only to allow magnetic powders, etc., to be dispersed in the magnetic layer with high dispersibility, but also to ensure good processability on calendering equipment (or good susceptibility to calendering), so that abrasion loss of the head, detachment of powders from the overall surface of the magnetic layer, etc., can be reduced. Even when the binder having such composition as set forth in the above publication is used, however, much difficulty is involved in preventing detachment of powders from the edges of tape to be referred to hereinafter. In this connection, it is to be understood that the results of studies made by the inventors teach that the use of resin having Tg of 50.degree. C. or higher and containing a polar group, especially, polyurethane resin makes it possible not only to prevent detachment of powders from the overall surface of the magnetic layer set forth in the above publication, but also to avoid detachment of powders from the edges of tape. When the above polyurethane resin having Tg of 50.degree. C. or higher and containing a polar group is used according to the above publication, however, there is a drop of running stability in a high-temperature environment, which in turn results in unavoidable inconvenience.
This also holds for JP-A 5-307734 precedent to this application, wherein polyester polyurethane resin of 69.degree. C. and 76.degree. C. is used in combination with polyester polyurethane resin of -27.degree. C. and -25.degree. C.